Herpes simplex virus 1 has afflicted more than 80% of the population and can cause diseases that range in severity from benign cold sores to encephalitis. Following replication in mucosal epithelial cells the virus establishes latent reservoirs in sensory neurons that innervate the infected tissues. Reactivation of the virus in these neurons upon stress or weakened immune responses is the source of virus in recurrent disease. To infect and persist in the host, HSV-1 has evolved strategies to counteract host antiviral responses. The immediate early protein of the virus Infected Cells Protein No 0 (ICP0) has a fundamental role in this process. In vivo, ICP0 is essential for successful colonization of the host, for virus replication, and for efficient reactivation of the virus from neuronal latency. During the early steps of the infection, ICP0 localizes in the nucleus where it activates viral gene transcription by blocking repressors of the viral genome, by inducing viral chromatin remodeling, and by inhibiting antiviral responses. The onset of virus replication triggers translocation of ICP0 to the cytoplasm, where it resides for the remainder of the virus replication cycle. The functions of ICP0 out of the nucleus have only scantily been explored. ICP0 in the cytoplasm interacts with the adaptor protein CIN85, a binding partner of the Cbl E3 ligase, which has a major role in cell surface receptor internalization, endocytic processing and protein sorting. The virus via ICP0 appears to subjugate the endocytic machinery Cbl/CIN85 to eliminate surface components, which could initiate harmful signals or could affect the ability of the virus to replicate and spread. Known manifestations of the ICP0/CIN85/Cbl interactions include the down- modulation of the epidermal growth factor receptor (EGFR) and of the viral entry receptor Nectin-1 from the surface of the infected cells. Nectin-1 was retained on the surface of cells infected with an ICP0-null virus and on the surface of the Cbl-knockdown cells after infection with the wild type virus causing a defect in virus spread. Based on these observations, the central hypothesis of this proposal is that HSV-1 via ICP0 co- opts the Cbl/CIN85 complex to promote endocytosis of cell surface factors, to modify signaling responses, for its benefit. We have designed two Specific Aims to investigate this hypothesis. In Aim 1, we propose to interrogate the interaction of ICP0 with the CIN85/Cbl complex and its significance during HSV-1 infection. In Aim 2, we propose to define the role of ICP0 in endocytosis mediated by the Cbl/CIN85 complex in infected cells. These studies are expected to delineate the importance of interaction of ICP0 with the Cbl/CIN85 endocytic machinery. This interaction could lead to alterations of the cell surfaceome that could impact intracellular signaling and intercellular communication. Overall, this could be a strategy by which the virus evades the host.